Although any desired micromechanical components can also be used, the present disclosure and the problem on which it is based are explained using silicon-based components.
DE 44 45 553 A1 describes a semiconductor acceleration sensor having a semiconductor substrate, a cantilever structure which is supported by the semiconductor substrate and has a movable electrode, arranged at a predetermined distance above the semiconductor substrate, as well as fixed electrodes which are arranged on the semiconductor substrate. A sensor section is formed by the movable electrode and the fixed electrodes and detects an acceleration by changes in a current between the fixed electrodes, which changes are caused by an offset of the movable electrode associated with the effect of the acceleration on the sensor section.
EP 0 990 911 A1 describes a micromechanical sensor based on the field effect transistor with a movable gate which is movable in a direction parallel to the substrate surface, the movement of the gate in this direction increasing or reducing the channel region overlapped by the gate in at least one MOSFET.
Micromechanical sensor apparatuses with a movable gate usually have evaluation circuits for detecting very small movements, which theoretically have an excellent signal-to-noise ratio and are therefore suitable for use in extremely miniaturized acceleration sensors, for example.
FIG. 3 shows a schematic cross-sectional view for explaining a known micromechanical sensor apparatus with a movable gate in a vertical cross section.
In FIG. 3, reference symbol 2 denotes a silicon substrate in which a drain region 3, a source region 4 and an intermediate channel region 7 of a field effect transistor are provided. A gate insulation layer 5, for example an oxide layer, is provided on the channel region 7. Reference symbol 6 is used to indicate surface charges on the insulation layer 5. A movable gate electrode 1 is movably arranged above the substrate 2 in a manner separated by an intermediate space Z.
The method of operation of such a micromechanical sensor apparatus with a movable gate is as follows: an external force moves the gate electrode 1 in the x,y direction and/or in the z direction. This movement changes the number of charge carriers within the channel region 7 and thus changes the resistance between the drain region 3 and the source region 4. This change in resistance can be detected either by applying a constant voltage and measuring the associated current or by injecting a constant current and measuring the associated voltage change.
A special feature of a sensor apparatus constructed in this manner is the increased noise for displacements in the x,y direction, which noise is observed in real applications. This increased noise can be largely attributed to parasitic leakage currents.